The present invention relates to a field-sequential type display device and display method for performing a display by synchronizing the switching of colors of light to be incident on an optical switching element with the inputting of display data of respective colors corresponding to a display image to the optical switching element, and also relates to a color-filter type display device and display method for performing a color display by synchronizing the incidence of white light on an optical switching element having color filters with the inputting of display data of respective colors corresponding to a display image to the optical switching element.
Along with the recent development of so-called information-oriented society, electronic apparatuses, such as personal computers and PDA (Personal Digital Assistants), have been widely used. With the spread of such electronic apparatuses, portable apparatuses that can be used in offices as well as outdoors have been used, and there are demands for small-size and light-weight of these apparatuses. Liquid crystal display devices are widely used as one of the means to satisfy such demands. Liquid crystal display devices not only achieve small size and light weight, but also include an indispensable technique in an attempt to achieve low power consumption in portable electronic apparatuses that are driven by batteries.
The liquid crystal display devices are mainly classified into the reflection type and the transmission type. In the reflection type liquid crystal display devices, light rays incident from the front face of a liquid crystal panel are reflected by the rear face of the liquid crystal panel, and an image is visualized by the reflected light; whereas in the transmission type liquid crystal display devices, the image is visualized by the transmitted light from a light source (back-light) placed on the rear face of the liquid crystal panel. Since the reflection type liquid crystal display devices have poor visibility because the reflected light amount varies depending on environmental conditions, transmission type color liquid crystal display devices are generally used as the display devices of personal computers displaying full-color images.
As the color liquid crystal display devices, TN (Twisted Nematic) type liquid crystal display devices using switching elements such as a TFT (Thin Film Transistor) are widely used. Although the TFT-driven TN type liquid crystal display devices have better display quality compared to an STN (Super Twisted Nomadic) type, they require a back-light with high intensity to achieve high screen brightness because the light transmittance of the liquid crystal panel is only 4% or so at present. For this reason, a lot of power is consumed by the back-light. Moreover, since a color display is achieved using color filters, a single pixel needs to be composed of three sub-pixels, and there are problems that it is difficult to provide a high-definition display, and the purity of the displayed colors is not sufficient.
In order to solve such problems, the present inventor et al. developed field-sequential type liquid crystal display devices (see, for example, T. Yoshihara et al., AM-LCD '99 Digest of Technical Papers, p. 185, 1999; and T. Yoshihara et al., SID '00 Digest of Technical Papers, p. 1176, 2000). Since such a field-sequential type liquid crystal display device does not require sub-pixels, it is possible to easily achieve a higher definition display compared to a color-filter type liquid crystal display device. Moreover, since the field-sequential type liquid crystal display device can use the color of light emitted by the light source as it is for display without using a color filter, the displayed color has excellent purity. Furthermore, since the light utilization efficiency is high, this device has the advantage of low power consumption. However, in order to realize a field-sequential type liquid crystal display device, a high-speed responsiveness (2 ms or less) of liquid crystal is essential.
In order to increase the speed of response of a filed-sequential type liquid crystal display device with significant advantages as mentioned above or of a color-filter type liquid crystal display device, the present inventor et al. are conducting research and development on the driving of liquid crystal such as a ferroelectric liquid crystal having spontaneous polarization, which may achieve 100 to 1000 times faster response compared to a prior art, by a switching element such as a TFT (Thin Film Transistor). In the ferroelectric liquid crystal, as shown in FIG. 1, with the application of voltage, the long-axis direction of the liquid crystal molecule is tilted. A liquid crystal panel sandwiching the ferroelectric liquid crystal therein is sandwiched by two polarization plates whose polarization axes are orthogonal to each other, and the intensity of the transmitted light is changed using the birefringence caused by the change in the long-axis direction of the liquid crystal molecule.
FIG. 2 illustrates an example of time chart of display control in a conventional filed-sequential type liquid crystal display device. FIG. 2(a) shows the scanning timing of each line of the liquid crystal panel, and FIG. 2(b) shows the ON timing of red, green and blue colors of the back-light. One frame is divided into three sub-frames, and, for example, as shown in FIG. 2(b), red light is emitted in the first sub-frame, green light is emitted in the second sub-frame, and blue light is emitted in the third sub-frame.
Meanwhile, as shown in FIG. 2(a), for the liquid crystal panel, image data writing scanning and erasing scanning are performed within a sub-frame of each of red, green and blue colors. However, the timings are adjusted so that the start timing of the writing scanning coincides with the start timing of each sub-frame, and the end timing of the erasing scanning coincides with the end timing of each sub-frame, and the time necessary for each of the writing scanning and the erasing scanning is set to a half of each sub-frame. During the writing scanning and the erasing scanning, voltages which are equal in magnitude and different in polarity corresponding to the same image data are applied to the liquid crystal panel. Moreover, the light emission time of each color is equal to the time of a sub-frame (see, for example, Japanese Patent Application Laid-Open No. 11-119189/1999).
Accordingly, the amount of light actually used for display is about a half of the amount of the emitted light. This is due to the fact that the time in which the light is transmitted from the liquid crystal panel as the optical switching element is about a half of the time of a sub-frame. More specifically, even after the erasing scanning, although the same image as that displayed after the writing scanning is displayed with very low brightness compared to the image after the writing scanning, this image is actually perceived as a “black image”, and therefore the time in which the light is transmitted through the liquid crystal panel is about a half of the time of a sub-frame.
Note that in this specification, scanning for obtaining a display image with high brightness is called “writing scanning”, while, scanning for obtaining an image with low brightness or a black image is called “erasing scanning”.
A field-sequential type liquid crystal display device has the advantages of high light utilization efficiency and reducing power consumption compared to a color-filter type liquid crystal display device. However, since it utilizes only about a half of the light from the light source for display as described above, there is a demand for a further improvement in light utilization efficiency. Similarly, in a color-filter type liquid crystal display device using a ferroelectric liquid crystal material, since the writing scanning and the erasing scanning are performed by respectively using a half of the time of each frame, there is a problem that only about a half of the light emitted by the light source is utilized for display.